Dimethyl ether (DME) is a versatile compound capable of being used as combustion fuel, cooking fuel, additive to liquefied propane gas, and intermediate for the production of other chemical compounds. The basic steps in the dimethyl ether synthesis from synthesis gas are as are as follows:CO+2H2→CH3OH  1)2CH3OH→CH3OCH3+H2O  2)
Equilibrium conversion may be increased if the water gas shift reaction (WGS) is also involved:CO+H2O→CO2+H2 In this case the net reaction is:3CO+3H2→CH3OCH3+CO2 
Dimethyl ether is produced industrially in a one step process in which the methanol synthesis, dehydration, and WGS steps are performed on the same catalyst bed reactor, or in a two step process in which the two or more steps are performed in two or more sequential reactors. The one-step process is preferable for thermodynamic and economic considerations. This process is known as the syngas-to-DME process. The reaction is favored by high pressures and low temperatures. In both processes carbon dioxide is produced in stoichiometric amounts along with dimethyl ether. A prevalent problem in dimethyl ether synthesis from synthesis gas is the selective separation of the reaction product from carbon dioxide, methanol, water and unreacted syngas.
Prior art technology for the separation of dimethyl ether from other products includes U.S. Patent Application No. 2010/0216897 titled “Process for the preparation of Dimethyl Ether”, which discloses a 2-stage scrubbing process that uses DME in a first zone and methanol in a second zone as solvents. The physical basis for the invention lies in the high solubility of carbon dioxide in DME, and the high solubility of DME in methanol. Dimethyl ether, methanol and water are condensed out of the initial gas stream while allowing the flow of carbon dioxide and unreacted synthesis gas. The carbon dioxide (still containing DME vapor due to the high solubility of CO2 in DME) is passed through a first DME-containing scrubbing unit which removes carbon dioxide. Subsequently the CO2-reduced stream is passed through a second unit containing methanol which removes remnant DME. Unreacted synthesis gas is sent back to the DME synthesis reactor.
U.S. Pat. No. 7,910,630 assigned to Haldor Topsoe discloses a separation method which utilizes a cooled solvent of a dialkyl ether of a polyethylene glycol to solubilize both carbon dioxide and DME after an initial condensation of methanol and water. The remaining CO2 and DME are selectively desorbed in a subsequent process. U.S. Pat. No. 7,652,176, also to Haldor Topsoe, discloses passing a product stream comprised of DME, methanol, CO2, and unreacted synthesis gas through an absorber comprised of 20-40% potassium carbonate to reduce the carbon dioxide levels below 500 ppm. An additional solid adsorbent comprised of zeolites, molecular sieves, or activated aluminas are used to bring down the levels below 1 ppm. Subsequent exposure to a distillation column affords a separation of the DME from the methanol/water mixture.
U.S. Pat. No. 6,458,856 assigned to Air Products, Inc. describes a scrubbing solvent that uses a mixture of dimethyl ether and methanol to separate DME/CO2 from unreacted synthesis gas after an initial condensation of water and carbon dioxide. The DME/CO2 mixture is subsequently flash vaporized and distilled. This mixture is said to be superior to scrubbing agents of pure water, pure methanol, or pure DME because of the high solubility of CO2 in the mixture and low vapor pressure of the mixture. U.S. Pat. No. 5,908,963 assigned to Haldor Topsoe discloses a separation of CO2/unreacted syngas from dimethyl ether, methanol and water via condensation. Methanol is then distilled out from the condensed mixture and recycled to be utilized as a DME scrubber.
U.S. Pat. No. 6,147,125 to Shikada et al. discloses a separation method which condenses methanol and water first, and passes through dimethyl ether, carbon dioxide, and unreacted synthesis gas. The syngas is recycled, while the DME/CO2 mixture is condensed out. A subsequent distillation separates the dimethyl ether form the carbon dioxide.
Chinese Patent No. 1085824A teaches the use of water or ethanol as scrubber solvent in the production of dimethyl ether from synthesis gas. Extraction conditions are said to be room temperature and pressure above 10 bar, with a volume ratio >=0.03 when water is used as an extractant and >=0.01 when ethanol is used as the extractant. Han et at (Chemical Industry and Engineering Progress, 2008-06) in an article titled “Separation process of dimethyl ether synthesized by one-step method from syngas” describe a separation process that utilizes an absorption column at 40° C. and 20 bar and that contains de-ionized water as the scrubbing solvent. The process is said to produce high purity DME after a large number of refluxes.